This invention relates to optoelectronic devices, and more particularly, to organic light emitting devices (organic EL devices). More specifically, the present invention relates to substantially stable organic EL devices which have relatively long operational lifetime, such as at least about 1,000 hours before their luminance drops to some percent of its initial value, such as about 50 percent of the initial luminance, and which devices are substantially stable at high temperatures, such as from about 70xc2x0 C. to about 100xc2x0 C.
An organic electroluminescent (EL) device can be comprised of a layer of an organic luminescent material interposed between an anode, typically comprised of a transparent conductor, such as indium tin oxide, and a cathode, typically a low work function metal such as magnesium, calcium, aluminum, or the alloys thereof with other metals. The EL device functions on the primary principle that under an electric field, positive charges (holes) and negative charges (electrons) are respectively injected from the anode and cathode into the luminescent layer and undergo recombination to form excitonic states which subsequently emit light. A number of organic EL devices have been prepared from a laminate of an organic luminescent material and electrodes of opposite polarity, which devices include a single crystal material, such as single crystal anthracene as the luminescent substance as described, for example, in U.S. Pat. No. 3,530,325, the disclosure of which is totally incorporated herein by reference. These types of devices are believed to require excitation voltages on the order of 100 volts or greater.
An organic EL device with a multilayer structure can be formed as a dual layer structure comprising one organic layer adjacent to the anode supporting hole transport, and another organic layer adjacent to the cathode supporting electron transport and acting as the organic luminescent zone of the device. Examples of these devices are disclosed in U.S. Pat. Nos. 4,356,429; 4,539,507; 4,720,432, and 4,769,292, the disclosures of which are totally incorporated herein by reference, wherein U.S. Pat. No. 4,769,292, the disclosure of which is totally incorporated herein by reference, discloses, for example, an organic EL device comprising three separate layers, a hole transport layer, a luminescent layer, and an electron transport layer, which layers are laminated in sequence and are sandwiched between an anode and a cathode, and wherein a fluorescent dopant material is added to the emission zone or layer whereby the recombination of charges results in the excitation of the fluorescent material. In some of these multilayer structures, such as, for example, organic light emitting devices described in U.S. Pat. No. 4,720,432, the disclosure of which is totally incorporated herein by reference, the organic light emitting device further comprises a buffer layer interposed between the hole transport layer and the anode. The combination of the hole transport layer and the buffer layer forms a dual-layer hole transport region, reference S. A. Van Slyke et al., xe2x80x9cOrganic Electroluminescent Devices with Improved Stability,xe2x80x9d Appl. Phys. Left. 69, pp. 2160-2162, 1996, the disclosure of which is totally incorporated herein by reference.
There have also been attempts to obtain electroluminescence from organic light emitting devices containing mixed layers, for example, layers in which both the hole transport material and the emitting electron transport material are mixed together in one single layer, see, for example, Kido et al., xe2x80x9cOrganic Electroluminescent Devices Based On Molecularly Doped Polymers,xe2x80x9d Appl. Phys. Lett. 61, pp. 761-763, 1992; S. Naka et al., xe2x80x9cOrganic Electroluminescent Devices Using a Mixed Single Layer,xe2x80x9d Jpn. J. Appl. Phys. 33, pp. L1772-L1774, 1994; W. Wen et al., Appl. Phys. Lett. 71, 1302 (1997); and C. Wu et al., xe2x80x9cEfficient Organic Electroluminescent Devices Using Single-Layer Doped Polymer Thin Films with Bipolar Carrier Transport Abilitiesxe2x80x9d, IEEE Transactions on Electron Devices 44, pp. 1269-1281, 1997. In a number of these devices, the electron transport material and the emitting material can be the same or the mixed layer can further comprise an emitting material as a dopant. Other examples of organic light emitting devices which are formed of a single organic layer comprising a hole transport material and an electron transport material can be found, for example, in U.S. Pat. Nos. 5,853,905; 5,925,980; 6,114,055 and 6,130,001, the disclosures of which are totally incorporated herein by reference. As indicated in the article by S. Naka et al., these single mixed layer organic light emitting devices are generally less efficient than multilayer organic light emitting devices. These devices, which include only a single mixed layer of a hole transport material, such as NPB (N,Nxe2x80x2-di(naphthalene-1-yl)-N,Nxe2x80x2-diphenyl-benzidine), and an emitting electron transport material, such as Alq3 (tris (8-hydroxyquinoline) aluminum), are believed to be unstable and to have poor efficiency. The instability of these devices is believed to be caused by the direct contact between the electron transport material in the mixed layer and the hole injecting contact comprised of indium tin oxide (ITO), which results in the formation of an unstable cationic electronic transport material, and the instability of the mixed layer/cathode interface, see H. Aziz et al., Science 283, 1900 (1999), the disclosure of which is totally incorporated herein by reference. In addition, the single mixed layer may result in high leakage currents and hence poor efficiency, see Z. D. Popovic et al., Proceedings of the SPIE, Vol. 3176, xe2x80x9cOrganic Light-Emitting Materials and Devices IIxe2x80x9d, San Diego, Calif., Jul. 21-23, 1998, pp. 68 to 73, the disclosure of which is totally incorporated herein by reference.
While recent progress in organic EL research has elevated the potential of organic EL devices for widespread applications, the operational stability of current available devices may in some instances be below expectations. A number of known organic light emitting devices have relatively short operational lifetimes before their luminance drops to some percentage of its initial value. Providing interface layers as described, for example, in S. A. Van Slyke et al., xe2x80x9cOrganic Electroluminescent Devices with Improved Stability,xe2x80x9d Appl. Phys. Lett. 69, pp. 2160-2162, 1996, and doping as described, for example, in Y. Hamada et al., xe2x80x9cInfluence of the Emission Site on the Running Durability of Organic Electroluminescent Devicesxe2x80x9d, Jpn. J. Appl. Phys. 34, pp. L824-L826, 1995, may perhaps increase the operational lifetime of organic light emitting devices for room temperature operation, however, the effectiveness of these organic light emitting devices deteriorates for high temperature device operation. In general, the device lifetime can be reduced by a factor of about two for each 10xc2x0 C. increment in the operational temperature. Moreover, at high temperatures, the susceptibility of the organic light emitting devices to degrade is increased as described, for example, in Zhou et al., xe2x80x9cReal-Time Observation of Temperature Rise and Thermal Breakdown Processes in Organic Leds Using an IR Imaging And Analysis Systemxe2x80x9d, Advanced Materials 12, pp 265-269, 2000, which further reduces the stability of the devices. As a result, the operational lifetime of these organic light emitting devices at a normal display luminance level of about 100 cd/m2 is limited, for example, to about a hundred hours or less at temperatures of about 60xc2x0 C. to about 80xc2x0 C., reference J. R. Sheats et al., xe2x80x9cOrganic Electroluminescent Devicesxe2x80x9d, Science 273, pp. 884-888, 1996, and also S. Tokito et al., xe2x80x9cHigh-Temperature Operation of an Electroluminescent Device Fabricated Using a Novel Triphenylamine Derivativexe2x80x9d, Appl. Phys. Lett. 69, 878 (1996).
This invention provides in embodiments organic light emitting devices with, for example, excellent operational lifetimes. The organic light emitting devices according to embodiments of the present invention can provide operational stability at high temperatures, such as, for example, an operational lifetime of several hundreds of hours, such as 1,200 hours at a high brightness of, for example, about 1,500 candelas per square meter (cd/m2) at temperatures of from about 70xc2x0 C. to about 100xc2x0 C., which corresponds to up to several thousands of hours, such as about 10,000 hours, for typical display luminance of about 100 cd/m2 at temperatures of from about 80xc2x0 C. to about 100xc2x0 C. Also, the organic light emitting device according to embodiments of the present invention demonstrate reduced increases in driving voltage, such as, for example, increases of not more than about 5 percent of the initial value of the driving voltage even after operation for about 250 hours at a temperature of about 90xc2x0 C., which is about 10 times smaller than increases in a number of prior art devices, such as for example, devices illustrated by Van Slyke et al.